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Full text of "Treatise On Applied Analytical Chemistry(Vol-1)"

ALLOYS  OF GOLD,  SILVER AND COPPER             291

II.  MEDIUM ALLOYS,    (a) Determination oj the gold and silver.   Two
quantities of 0-5 gram are cupelled under the conditions given for gold-
copper alloys but at a somewhat higher temperature, and with no added
silver.    The amount of lead to be added is calculated on the basis of the
total gold and silver contents, one-fourth of the quantity indicated by
the table for the cupellation of gold and three-fourths of that given for the
cupellation of silver, being taken.

Example : If the preliminary assaj^ gives Au 220 and Ag 680 per 1000,
the total fineness is 900. For this value the table for the cupellation of
gold would indicate 5 grams of lead, so that 1-25 is taken, and the table
for the cupellation of silver would indicate 3-5 grams of lead, so that 2-6
(=3 X 3-5/4) is taken ; the total amount of lead taken is thus 1-25 +
2-6 = 3-85 grams.

The buttons obtained are weighed together and subjected to parting.
The gold is given by the total weight of the two cornets and the silver by
difference (button minus gold).

III.  POOR ALLOYS.   Two 0*5 gram samples are cupelled as described
for the cupellation of silver and with the amounts of lead there prescribed ;
the temperature is, however, kept somewhat higher, especially if the fineness
of the gold exceeds 50 per thousand.    The sum of the weights of the two
buttons gives the silver + gold.    To separate the silver, the buttons are
parted, bearing in mind that, if the fineness of the gold is not more than
5 per 1000, the two buttons should be parted in the same flask, and that,
for values exceeding 20 per 1000, the buttons should be rolled and reheated
at low redness ;   further, that, before decanting the acid from the flask,
the liquid should be given a rotary motion, so that the gold dust collects
at the bottom; that the third treatment with acid should be omitted and
that great care is necessary to avoid loss during the descent of the gold
dust into the crucible.

If the gold content does not exceed 60-80 per 1000, the silver may be
determined with greater exactitude by the Gay-Lussac method (see Silver
and its Alloys).

*
* *

Crude gold contains considerable quantities of impurities, especially silver,
copper, lead, bismuth, tin, antimony, arsenic, etc. Thus, gold obtained by
amalgamation varies from 865 to 970 fine, whereas that given by the Siemens
process has a fineness of 890-900 and contains, besides silver, only traces of
copper and lead ; gold precipitated by zinc is 600-700 fine and contains con-
siderable proportions of zinc, lead, iron and copper.

Refined gold reaches the fineness 993-999, but always contains small quan-
tities of silver.

Gold is used more especially alloyed with copper for jewellery, coinage,
medals, etc. The legal fineness of the Italian gold coinage is goo _+ i and that
of British coinage 916-66. With jewellery, plate, etc., the fineness may vary
from 920 to 500.

Gold-silver and gold-silver-copper alloys are also largely used, more especially
to obtain special effects in articles of jewellery (green gold : 750 Au, 250 Ag ;
red gold : 750 Au, 200 Ag, 50 Cu ; white English gold : 750 Au, 170 Ag, 80 Cu,
etc.).m three bottles to receive : (i) the first two lots of acid, rich in silver nitrate,